Current demands for high density and performance associated with ultra large scale integration require submicron features, increased transistor and circuit speeds and improved reliability. Such demands require formation of device features with high precision and uniformity, which in turn necessitates careful process monitoring, including frequent and detailed inspections of the devices while they are still in the form of semiconductor wafers.
Conventional in-process monitoring techniques employ a particle beam apparatus, such as a scanning electron microscope (SEM), for defect review. The SEM scans the surface of a specimen with an energetic particle beam; e.g., an electron beam having an energy of about 400 eV to about 1 keV. The impact of the particle beam on the surface of the specimen causes electrons of the particle beam to be deflected, or "backscattered", by atoms of the specimen with energies close to that of the particle beam (i.e., about 400-1000 eV). These deflected electrons are referred to as backscattered electrons. Electrons released from the surface of the specimen due to the impact of the particle beam, with energies of about 50-100 eV, are referred to as secondary electrons. It is well known that since the backscattered electrons are deflected by atoms of the specimen, they provide information relating to the material composition of the specimen (e.g., density information). On the other hand, the secondary electrons are released from the specimen corresponding to the slope of the surface of the specimen, so secondary electrons provide topographical information, i.e., the distribution of secondary electrons will vary with the slope of the surface of the specimen. Thus, images of a semiconductor wafer under review showing material contrast can be generated after detecting backscattered electrons, and images of the wafer showing the bottoms of trenches and contact holes, as well as particles and other surface features, can be generated after detecting secondary electrons. Furthermore, certain attributes of a defect may be apparent only in an image generated by a mix of both backscattered and secondary electrons.
FIG. 1 depicts a prior art SEM for producing such images, wherein a primary beam 11 is directed at a wafer 12 to produce backscattered electrons BSE and secondary electrons SE, which are detected by a single detector 13. Backscattered electrons BSE tend to deflect from wafer 12 at acute angles, while secondary electrons SE are dispersed at many angles. Secondary electrons SE which impinge detector 13 simultaneously with backscattered electrons BSE are selectively repelled by an electrode in the form of a grid 14 provided near the entry side of detector 13 to produce the desired images of wafer 12.
In practice, a switch 15, typically a potentiometer, is manually adjusted to negatively bias grid 14 such that an image of the desired combination of backscattered and secondary electrons is produced. For example, a negative potential of about the same energy as secondary electrons SE is applied to grid 14 via switch 15 to repel most secondary electrons SE, thus allowing easier detection of backscattered electrons BSE, which are not significantly repelled by the negative bias on grid 14 because they are of a substantially higher energy. Then, grid 14 is switched off and an image is generated from secondary electrons SE, since when grid 14 is not biased, mostly secondary electrons impinge on detector 13.
The detection apparatus of FIG. 1 suffers from the disadvantage of not providing simultaneous viewing of images generated as a result of the detection of backscattered electrons BSE and secondary electrons SE; that is, the two or more respective images are normally produced sequentially (by varying the potential on grid 14), so that only one image can be seen by the operator at a time.